When infrared optical systems are taken into consideration, a number of physical properties in connection with this kind of infrared technology must be regarded. The limited amount of light energy transmitted through the objective constitutes additional difficulties to infrared autofocuses in comparison with visible light autofocuses. In infrared optical systems like the above mentioned, the limiting device is the detecting unit and its performance and efficiency characteristics. This is one of the reasons explaining that the attempts at constructing automatic infrared image analysing focuses have not succeeded in the past.
An article of Frank R. Boddeke et al. in the publication Bioimaging, 2, 193–203, (1994) discloses an algorithm for autofocusing including usage of a focus function, which algorithm constitutes a basis for image analysing autofocuses. Single and multiple use of simple filters are described as well as noise reducing methods in practice.
Due to the above mentioned additional difficulties associated with infrared autofocusing apparatus, the complexity of prior known systems has increased to an unacceptable level when attempts have been made at managing the low energy level of infrared light. The major problem related to such infrared systems is the low f-number. Infrared systems require a low f-number and as a consequence of this number. Infrared systems require a low f-number and as a consequence of this requirement, the signal-noise-ratio is low and hence a large number of acquired images must be analysed in order to obtain sufficient resolution of the viewed object. Processing capacity in such objectives is limited due to the additional costs associated with higher processing capacity and the cost will finally fall on the end consumer. Until today, no successful solutions of this problem have been made.
Earlier a focus window has been chosen as a central field of the image detecting area. This central field is in practice situated perpendicularly to the optical axis. Thus, this used to be the traditional method of proceeding and it is still used by most of the prior known active autofocusing systems. However, only a minimal fraction of the actual area of the detected image is utilised by this method and desirable would be to use a larger or at least the most suitable area of the detected image as focus window.